Solid to liquid transformations by technology described in Lasko patents U.S. Pat. No. 5,584,419 and U.S. Pat. No. 7,755,009 require inductor coil forms that often impede material flow. Solid or particulate form electrically nonconductive materials are presented to one surface of an inductively heated perforated susceptor for melt transformation upon passing to the other side by gravity flow or mechanical pressure. When the susceptor form is a disc, it acts as a face of a cylindrical container for the process material. A cone form susceptor acts as a conical end of a cylindrical container. A cylinder form susceptor is a portion of the cylindrical container. These shapes are necessarily fully radial to accomplish an evenly distributed coupling of the magnetic field. The objective of the inductor coil design for this melting process is to distribute the magnetic field intensity in proportion to the volume flow over the surface of the susceptor. Efficient transfer of energy to the susceptor requires placement of the individual inductor elements in close proximity to the susceptor surface. The number of elements [off-set concentric turns or spiral turns] per unit area of the susceptor surface is varied to distribute the magnetic field intensity and resulting energy transfer from the inductor coil to the susceptor. These variations control the influence of the inductor coil magnetic field edge effect and inter-turn deviation [flux leakage].
Sheets of industry standard staggered round hole perforated steel are used to construct susceptors of disc, cone and cylinder form. The size and number of perforations in the susceptor are chosen to maximize the surface area of the susceptor for thermal conduction to the process material, while restricting open area to preserve thin sheet strength and adequate cross sectional area for even induced current flow. The thermal conductivity and temperature variable viscosity of the process material further defines the hole size. An open area of approximately 50% meets this requirement for most materials. The material must flow through the susceptor in unimpeded volume related to the energy transferred at any point on the susceptor to impart a homogeneous material temperature.
Processing different materials in the same apparatus requires purging the previous material with the new material. Additional surfaces of inductor coil supports and the coil occupied area add to the volume of material lost to this process. Lesser viscosity materials in gravity flow will not adequately displace materials of greater viscosity. Removing the inductor and susceptor for chemical cleaning is not an attractive alternative. The process start and stop interval is lengthened by the total thickness of the inductor coil and susceptor assembly. Because the susceptor is the material containment vessel or a part there of, support for this item in the apparatus is complicated by the necessary close proximity position of the inductor coil.
This invention provides a method of meeting these physical and electrical requirements by direct placement of the inductor coil on the susceptor surface and perforating the inductor coil with axis and diameter coincident holes. The hydraulic pressure required to pass material through this thermal interface is reduced to that of the susceptor alone. The inductor coil does not need to be separately supported in the material flow path. Similar materials can be processed with minor volume displacement of the previous material in the apparatus. Extraction of the integral inductor-susceptor for chemical cleaning is made practical by requiring only the removal of an electrical connection and striping the surface of a single unit of simple form.
When the adjacent inductor coil material is axis coincidentally perforated, its electrical conducting cross section is diminished. The resistance of the total remaining conductor cross-section must remain low enough to support the desired amount of high frequency current having electrical energy losses that are thermally transferable to the process material. The thickness of the inductor coil is increased to preserve the required minimum cross section.
The inductor is made integral with the susceptor by direct placement on an electrically insulated susceptor surface. This bond provides an accurate and mechanically stable orientation of the inductor in closest proximity of the susceptor. This is achieved in one embodiment of the invention by plating the inductor coil on one or both surfaces of a porcelain enamel coated perforated steel disc. The perforated sheet steel disc is etched to radius the holes edges and decarburize the surface. The entire disc surface and holes are coated with 0.009″ of porcelain enamel. The disc is electroless copper plated, pattern masked, etched, striped, electroplated, and refired. The coefficient of thermal expansion of the steel disc susceptor, porcelain enamel coating, and copper overlay are close enough to maintain an effective bond for typical maximum process temperature excursions of 400° F.
The process residency time for most thermoplastic materials is a few seconds. Power applied at 20 to 50 watts/sq.″ will melt most thermoplastic materials at gravity pressure on the susceptor surface. The frequency of the power applied to the inductor coil is 40 to 100 KHz. The process temperature can be precisely controlled by placing a thermocouple on the susceptor to signal a controller for modulating the high frequency power applied to the inductor.